1) The desert(s) in Indus Valley region are roughly the same (or maybe even little bit less pervasive) as those in North America.

2) Non-dual understanding of reality (what you call monism, I suppose) and monotheistic religion are definitely not the same. Where polytheistic religion is an appropriation of animism in order to build a hierarchic social structure, monotheistic religion is an appropriation of non-dualism in order to do the same (or simply to curb the disruptive influence non-dualism has on all empires).

3) Scriptures and theologies are already religious artifacts that reflect the local culture. The "core" is interspersed throughout it. I'm not familiar with Avesta to the extent where I could draw a clear cut parallels with Vedic Hinduism or Judaism, but the latter two exhibit uncanny similarities --Ten Commandments vs (especially) Yamas (but also Niyamas); Mahavakyas vs Exodus 3:14 or Psalm 82:6; etc.